Allan:

I base my findings on the performance of active vs passive crossover networks on over 30 years in the professional audio industry in designing, installing and using speaker systems of both types. Ten of these years was spent at Altec Lansing and JBL Professional, both premier speaker manufacturers with large R&D departments. At Altec in particular, I delved very deeply into this area.

In my experiments, active crossovers always sounded cleaner, with much better dynamic performance since the signal did not have to pass through highly reactive components (inductors and capacitors) which greatly increase the complexity of the load presented to the power amplifier. Additionally, any "voicing" (equalization implemented in the passive crossover to suit the traits of the drivers) involves complex combinations of capacitors, inductors and resistors - the presence of resistors especially wastes power - that's what resistors are for, to limit current. This equals wasted power that would otherwise be directed to the speaker drivers.

The effects of a passive crossover in the signal path can be seen and measured with ocilloscopes and distortion analyzers. Just passing a square wave through a passive crossover and viewing the output presented to the speaker drivers is in it's self an informative exercise.

The use of an active crossover allows direct amplifier to speaker driver connection - this makes the power amplifier's life much easier because it only has to deal with the back EMF from the speaker drivers, without the added burden of inductive and capacitive reactance from a passive crossover network's components. Depending on the amplifier design, this reduces distortion and allows the amplifier to simply sound better. One of the reasons for the popularity of "bi-wiring" and "passive bi-amping" is the fact that with passive crossovers, the signal from one section of the crossover can modulate the other, causing distortion. This band-aid approach is unnecessary in an active setup since the drivers are completely isolated.

Additionally, the inductors necessary to implement a low pass filter to feed the woofer present a significant series resistance. It's literally like placing a resistor in-line with your speaker wires or using a thin-gauge speaker cable! You would never think of doing this, yet this is exactly what is happening inside a speaker with a passive crossover by virtue of the series resistance presented by the inductor in the woofer circuit. This is the very place you don't want resistance since it destroys the damping factor!

Is an active crossover always the only way a speaker should be designed? Of course not!! In the vast majority of instances, active crossovers are simply impractical for complexity and cost reasons. It takes a serious amount of effort and the proper equipment to properly sort out and set up in an optimum way an active system. The expertise to do this properly is very rare in the audio field.

I'm still tweaking with my own system after 30 years!! Active crossover use allows the speaker system to sound just about any way you wish by changing the crossover frequencies, slopes, phase and amplitude. The variations are endless - and the possiblilties for screw-ups are also endless.

This is why for the vast majority of users, a turn-key system using passive crossovers is better from a practical standpoint, even though this approach trades ultimate performance for ease of setup and lower cost.

There has been a recent trend toward professional monitor speakers that use built-in power amplifiers and active crossovers that incorporate crossover duties with intricate equalization and phase compensation that optimises the overall design toward really unbelievable sonic quality. One such speaker that I am very impressed with is the Mackie HR 824, which is being used by quite a few professional studios and film post production houses here in Hollywood, CA.

Speakers like the Mackie HR 824 retain turn-key practicality, yet benefit from the sonic superiorities of active crossovers with optimum equalization for the speaker driver/cabinet combination.

You should really try to hear these speakers if you get a chance: they show very convincingly what an optimum implementation of active crossovers can do in a single package design.

The future trend of "digital input" speakers (speakers which accept direct digital inputs from a signal source and incorporate DSP functions such as automatic room equalization) will have to incorporate active crossover networks (the active crossover will simply be part of the DSP processing) since the equalization functions would be impossible to implement in a passive design. These speakers will benefit greatly from the fact that the active crossover and power amplifiers are incorporated into, and optimised for, the overall speaker design.

Thankfully, as DSP chips find their way into more speaker designs, we can begin to look forward to a future where the benefits of active crossovers will begin to trickle down to the average consumer level and inefficient passive crossovers will be relagated to the dust bin of Hi-Fi history.

[This message has been edited by soundhound (edited October 09, 2004).]